1. Field of the Invention
The present invention relates to a double walled pipe, for example, a gas pipe or the like, having a double walled pipe structure, which is suitable for a jacking method in which an object is propelled under the ground to be buried underground, and a jacking method thereof, and further to a pipe end structure of a leading pipe which is used for leading the double walled pipe when it is propelled underground.
2. Description of the Related Art
It happens that as a pipe which is buried underground for transporting, for example, the high pressure gas, a double walled steel pipe is used. In such a double walled steel pipe, there is slushed the concrete or the like the space between the inner pipe and the outer pipe to maintain the strength of the pipe. Further, to bury such a double walled steel pipe under the ground, there may be adopted a so-called jacking method. According to the jacking method, burying of the double walled steel pipe is conducted in such a manner that a predetermined shaft is beforehand dug to propel the double steel pipe from the side wall of the shaft in the longitudinal direction, but not to bury the double walled steel pipe from the ground through open-cut of an elongated groove extended along the burying path.
FIGS. 35(A) and 35(B) are a side elevation and a section view, respectively, showing by way of example the conventional double walled steel pipe which will be used in adoption of the jacking method as described above.
Referring to FIGS. 35(A) and 35(B), a double walled steel pipe 31 basically comprises an inner pipe 33, an outer pipe 35 and a concrete 37 which fills up the space between the inner pipe 33 and the outer pipe 35. Where a front end portion 33a and a back end portion 33b of the inner pipe 33 are not covered by the concrete 37 and thus exposed.
There are prepared a plurality of pieces of such double walled steel pipe 31, and those are propelled underground in a direction as shown by the arrow in the figure. At that time, burying of those pipes is implemented in the following manner. A back end plane 33d of the preceding double walled steel pipe 31 and a front end plane 33c of the subsequent double walled steel pipe 31 are coupled to each other and welded, and then the welded portion is fitted with one having a ring configuration in which a ring is segmented into a plurality of pieces, a back end plane 35b of the outer pipe 35 of the preceding double walled steel pipe 31 and a front end plane 35a of the outer pipe 35 of the subsequent double walled steel pipe 31 are jointed each other, and the thrust is applied to the back end plane 33d of the inner pipe 33 of the subsequent double walled steel pipe 31 and the back end plane 35b of the outer pipe 35 of the subsequent double walled steel pipe 31 so that the subsequent double walled steel pipe 31 may propel in an arrow direction shown in FIG. 35 (A). This process is repeatedly conducted over and over. As a result, the coupled double walled steel pipes 31 are buried under the ground.
The use of the above-mentioned conventional double walled steel pipes, however, suffers from the following drawbacks. The propulsion of the double walled steel pipes may cause the back end plane 33d of the inner pipe 33 to be bent or injured, for example. Thus, prior to welding this together the inner pipe of the subsequent double walled steel pipe, there will be needed a process for reshaping the back end plane 33d. Further, the propulsion of the double walled steel pipes may cause the internal stress to be derived on the inner pipe 33. Thus, there is a fear such that for example, pinholes, clacks or the like will occur on the inner pipes, while the double walled steel pipes are buried underground and used.
In the jacking method adopting the above-mentioned double walled steel pipes 31, it is preferable from the view point of strength to fill up with the cement milk the space between the inner pipe 33 and the outer pipe 35 (or the fitted one in a ring shape) at a coupling portion of the double walled steel pipe 31 to the double walled steel pipe 31. However, it would be difficult to completely fill up the space without any gap. Thus, it happens that steel segments are inserted in a circumferential shape, without conducting filling up, and then the double walled steel pipes are propelled. This case will, however, involve the problem as to the strength of the coupling portion of the double walled steel pipes.
Further, as another jacking method of burying the double walled pipes, there is known a method in which only the outer pipe is propelled under the ground, in order to prevent the occurrence of flaws on the inner pipe through which gas or water will flow, and thereafter the inner pipe is fed or forced into the outer pipe, and then the concrete or mortar is injected into the gap between the inner pipe and the outer pipe to fix the inner pipe and the outer pipe together. According to this method, as mentioned above, there are needed two processes, one of which is for propelling the outer pipe under the ground, and another for feeding or forcing the inner pipe into the propelled outer pipe. Thus, according to this method, it takes much time, and in addition the cost for construction will be increased. Therefore, according to this method, from the view point of the cost, it is common that a concrete pipe (Hume pipe) is used as the outer pipe.